Wire-wound sensor componentry for mass flow meters

ABSTRACT

Microprocessor-based thermal dispersion mass flow meters (i.e., thermal anemometers) are described that use temperature sensing elements in its flow sensor probe(s) in addition to the two elements commonly used. Such systems allow for automatically managing changes in gas selection, gas temperature, gas pressure, and outside temperature. One mass flow meter described has a flow sensor with four temperature sensing elements, wherein one pair is provided in a temperature sensor probe and another pair in a velocity sensor probe. Another variation operates without a separate temperature sensor probe and integrates all function into a single three-sensor probe. Such a device may also be used in conjunction with a one- or two-sensor temperature probe.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of InternationalApplication No. PCT/US2012/056664, filed Sept. 21, 2012, which isincorporated by reference herein in its entirety and for all purposes.

FIELD OF THE INVENTION

This invention relates to mass flow meters, in configurations identifiedto enable computational modeling for use with different fluids andmethods of use in connection with such modeling.

BACKGROUND

Thermal dispersion mass flow meters directly measure the mass flow rateof single-phase pure gases and gas mixtures of known composition flowingthrough pipes or other flow conduits. They also have application tosingle-phase liquids of known composition. In most of the following, itis assumed that the fluid is a gas, without the loss of applicability toliquids.

The mass flow rate of a fluid (defined by its average velocitymultiplied by its mass density multiplied by the cross-sectional area ofthe channel through which the flow travels) is a measured quantity ofinterest in the control or monitoring of most practical and industrialapplications, such as any chemical reaction, combustion, heating,cooling, drying, mixing, fluid power, etc. For such purposes, gasesmonitored by industrial thermal dispersion mass flow meters typicallyinclude: air, methane, natural gas, carbon dioxide, nitrogen, oxygen,argon, helium, hydrogen, propane, and stack gases, as well as mixturesof these gases and mixtures of hydrocarbon gases.

Generally speaking, a thermal anemometer (alternatively referred to as athermal dispersion mass flow meter or simply as a mass flow meter) isused to measure the mass velocity at a point or small area in a flowingfluid—be it liquid or gas. The mass velocity of a flowing fluid is itsvelocity referenced to standard (or normal) temperature and pressure.The mass velocity averaged over the flow channel's cross-sectional areamultiplied by the cross-sectional area is the standard (or normal)volumetric flow rate through the channel and is a common way ofexpressing the total mass flow rate through the channel.

The thermal anemometer is sometimes referred to as an immersible thermalmass flow meter because it can be immersed in a flow stream or channelin contrast to other thermal mass flow meter systems, such as those thatsense the total mass flow rate by means of a heated capillary tubemounted externally to the flow channel.

The first general description of a thermal anemometer is attributed toL. V. King who, in 1914, published “King's Law” revealing how a heatedwire immersed in a fluid flow measures the mass velocity at a point inthe flow: King, L. V. 1914, “On the convection of heat from smallcylinders in a stream of fluid: Determination of the convectionconstants of small platinum wires with application to hot-wireanemometry.” Phil. Trans. Roy. Soc. A214: 373-432. King called hisinstrument a “hot-wire anemometer.”

Early applications of this technology were hot-wire and hot-filmanemometers and other light-duty thermal dispersion flow sensors used influid mechanics research and as light-duty mass flow meters and pointvelocity instruments. It was not until the 1960s and 1970s thatindustrial-grade thermal dispersion mass flow meters emerged that couldsolve the wide range of general industry's more ruggedized needs fordirectly measuring the mass flow rate of air, natural gas, and othergases in pipes and ducts.

Thermal dispersion mass flow meters measure the heat convected into theboundary layer of a fluid (e.g., liquid or gas) flowing over the surfaceof a heated velocity sensor immersed in the flow. Since it is themolecules of the gas that bear its mass and carry away the heat, thermaldispersion mass flow meters directly measure mass flow rate. In aconstant-temperature mode of operation, the “heated” sensor (as commonlyknown) incorporated in the design is maintained at an average constanttemperature above the fluid temperature. The temperature differencebetween the flowing fluid and the heated sensor results in an electricalpower demand in maintaining this constant temperature difference thatincreases in proportion to the fluid mass flow rate that can becalculated. In another approach, some thermal anemometers operate in aconstant-current mode in which a constant current or power is applied tothe heated sensor and the fluid mass flow rate is calculated from thedifference in the temperature of the heated sensor and the fluidtemperature sensor, which decreases as mass flow rate increases.

Thermal anemometers may have greater application to gases, rather thanliquids, because their sensitivity in gases is higher than in liquids.However certain examples described herein may be equally applicable tomass flow meters for use with liquids.

Many of the mass flow meters currently known may have shortcomings, someor all of which may be addressed by the present disclosure. For example,because the parts of the heated sensor of known thermal anemometers arenot sufficiently reproducible (i.e., dimensionally or electrically),known thermal anemometers require multi-point flow calibration ofelectrical output versus mass flow rate, in the actual fluid with whichthey will be used and within the actual ranges of fluid temperature andpressure of the particular application. With such a multi-point flowcalibration, some level of flow measurement accuracy may be attainable,however the accuracy is only be applicable to the particular fluid usedfor calibration only within the narrow ranges of fluid temperature andpressure within which the calibration was conducted.

For industrial applications, the separate heated velocity and fluidtemperature sensors are typically enclosed in a protective housingshell. Sometimes, the heated sensor is inserted into the tip of thehousing shell and surrounded by a potting compound, such as epoxy,ceramic cement, thermal grease, or alumina powder. In such systems,“skin resistance” and stem conduction are two major contributors tonon-ideal behavior and measurement errors. The so-called “skinresistance” is the electrical analog of thermal resistance occurringbetween the encased heated sensor and the external surface of thehousing exposed to the fluid flow. Hot-wire thermal anemometers havezero skin resistance, but thermal anemometers with a housing shell dohave some skin resistance. The use of a potting compound substantiallyincreases the skin resistance because such potting compounds have arelatively low thermal conductivity and are relatively thick.

Skin resistance (in units of degrees Kelvin per watt) results in atemperature drop between the encased heated sensor and the externalsurface of the housing that increases as the electrical power suppliedto the heated sensor increases. Skin resistance creates a “droop” anddecreased sensitivity in the power versus fluid mass flow ratecalibration curve, especially at higher mass flow rates. The so-calleddroop is difficult to quantify and usually varies from meter to meterbecause of variations in manufacturing repeatability and ininstallation. The ultimate result of these skin-resistance problems isreduced accuracy. Furthermore, the use of a surrounding potting compoundcan create long-term measurement errors caused by aging and by crackingdue to differential thermal expansion between the parts of the heatedsensor.

Accordingly, the highest quality heated sensors have a skin resistancewith a low numerical value that remains constant over the long term. Ofall known sensor configurations, the most successful at managing thesetradeoffs has been produced by the assignee hereof, Sierra Instrumentsin U.S. Pat. Nos. 5,880,365; 6,971,274; 7,197,953 and/or 7,748,267, thedisclosures of which patents are incorporated herein by reference intheir entirety.

Velocity sensor probes constructed as such may be known as “dry” sensorsin contrast to velocity sensors fabricated with potting cements orepoxies that are wet when mixed. As discussed, these “wet”velocity-sensor systems suffer long-term stability and other qualityissues due to changes in the potting compound. With regards to thetemperature sensors, degradation of any potting material incorporated intemperature sensor probes may only change response time, which may be arelatively minor effect, and as such temperature sensor probes mayemploy any convenient construction.

A significant source of potential error in either the temperature sensorprobe and/or velocity sensor probe relates to heat conduction along theprobe stem. For example, stem conduction causes a large fraction of theelectrical power supplied to the heated sensor to be lost through thestem of the heated sensor, down the housing, lead wires, and otherinternal parts of the heated sensor and ultimately to the exterior ofthe fluid flow channel. Stem conduction couples the electrical powersupplied to the encased heated sensor to the ambient temperature outsidethe channel. Typically, if the ambient temperature decreases, stemconduction increases; if ambient temperature increases, the conductiondecreases. In either case, as ambient temperature changes, stemconduction changes, and measurement errors occur. Similarly, stemconduction is responsible for errors in the encased fluid temperaturesensor's measurement because the fluid temperature sensor also iscoupled to the ambient temperature in this manner. Mass flow metersknown in the art do not account for stem conduction in sufficient mannerto achieve the measurement accuracy as may be desired in certainapplications.

Accordingly, the examples described herein may provide systems andmethods for measuring mass flow of a fluid with improved performance,including (but not limited to) the ability to meter different fluidswithout requiring flow calibration specific to the fluid or conditionsbeing monitored, as well as the ability to account for mode(s) of stemconduction heretofore unrecognized and, thus, obtain measurements withincreased accuracy.

SUMMARY

Examples of thermal dispersion mass flow meters (interchangeablyreferred to herein as thermal anemometers or mass flow meters) aredescribed, which may include “secondary” temperature sensing elements inone or more of their flow sensor probe(s). Such “secondary” temperaturesensing elements may be provided in addition to the primary sensingelements. In some examples, the primary sensing elements may include theheated sensor in a velocity probe and the non-heated sensor in atemperature probe, typically located distally with respect to thevelocity probe. Systems and methods according to the present disclosuremay allow for automatically managing changes in gas selection, gastemperature, gas pressure, and outside temperature, as will be furtherdescribed.

The subject mass flow meters may include one or more flow sensor probeswith a plurality of Resistance Temperature Detector (RTD) temperaturesensing elements. In certain examples, each of the velocity sensor probeand the temperature sensor probe, if present, may include two or moreRTD elements. As such, some embodiments of the present invention mayinclude four or more RTD elements, which may be operatively configuredto achieve a desired measurement accuracy. Systems according to thepresent invention may offer performance with accuracy as high as fromabout 1% to about 2% of reading (as opposed to full scale) over massflow rate ranges from about 10% to about 100% of full scale (or largerrange) and over a wide range of fluid temperatures and pressuresencountered in field applications (e.g., about +/−10 to 25 deg. K and+/−2 to 4 bar, respectively, generally referenced to their values atflow calibration) and for any of a number of commonly used fluids (e.g.,most “clean” gases, including air, methane, Ar, CO₂, He, N₂, O₂, C₂H₈,and mixtures of these components). Embodiments of the invention mayoffer high accuracy performance for a gas or gas mixture even when flowcalibration is advantageously performed with a single inexpensivesurrogate gas operated at inexpensive conditions (e.g., air at ambientconditions).

In one example, a first pair of RTD elements may be provided in thevelocity sensor probe of a mass flow meter, and a second pair of RTDelements may be provided in the temperature sensor probe of the massflow meter. Each of the RTD elements in the first pair and/or in thesecond pair may be arranged in a spaced apart configuration, as will befurther described, to facilitate measurements according to the examplesdescribed. Another variation may be configured without a separatetemperature sensor probe, and the functionality of the velocity probeand the fluid temperature probe may be integrated into a singlethree-sensor probe. In yet other examples, such integrated three-sensorprobe may be used in conjunction with an additional one- or two-sensorfluid temperature probe.

In a coordinated system, mass flow meters according to the presentdisclosure couple the flow sensor hardware with microprocessor-basedelectronics programmed with algorithms that manage changes in gasselection, gas temperature, gas pressure and outside temperature.Multivariable versions provide analog and digital outputs of mass flowrate, gas temperature, and (optionally) gas pressure. A selection ofsophisticated digital communication protocols commonly used by industrymay also be made available.

In reference to the temperature sensor probe, the data collected fromthe secondary sensor may be used to account specifically for conductionof heat into or out of the probe. As such, it is desirable that thedistance between the temperature sensors in the temperature probe ismaximized (given all other fit constraints) in order to offer thegreatest temperature spread/differential and thereby provide better dataresolution and accuracy.

In reference to the velocity sensor probe, in one embodiment of thepresent invention, a thin-film RTD (TFRTD) sensor is not used for theheated sensor. Instead, a wire-wound heated RTD sensor is employed.Important aspect(s) regarding the use of a wire-wound RTD in place of athin-film RTD will become apparent in view of the discussion of thecomputational models possible with such configuration. Further, thesecondary sensor of the velocity sensor probe may be placed adjacent theproximal end of a heated length of the wire-wound RTD sensor. Thedistance between the secondary sensor and the heated length isadvantageously minimized The separation between the secondary sensor andthe heated length (e.g. the distance between the two) may be less thanabout three diameters of the probe to satisfy assumptions made for useof the computational models described below. In certain examples, thedistance may be about two diameters of the probe, or in other examples,the distance may be about one diameter of the probe. Indeed, the activeregion of the secondary sensor may be in contact with the heated length.Then, with such spacing options, the sensor data is variously usedaccording to methods described herein. As such, the importance of thesensor spacing in the velocity sensor probe will be appreciated in viewof the computational models adapted to be used in conjunction with thehardware (e.g. sensor probes) described herein.

If the secondary sensor is positioned with its active area in contactwith the proximal end of the heated length, its temperature data can bedirectly used as the boundary condition for the proximal end in thesolution for the differential equitation shown below as Equation (1)used in system analysis. If separated by a distance, or gap, thetemperature measured provides this boundary condition to the solution bymeans of nodal analysis (included in such analysis are Finite ElementAnalysis (FEA) and other known methods such as electrical analog models)or by differential equation analysis (with ordinary or partialdifferential equations linked together via their boundary conditions).

A model is provided for the axial temperature distribution T₁(x) for aheated control volume (alternatively referred to as the “heated length”)of a velocity sensor comprising the heated winding, its binder/coatingand the insulating substrate upon which it is wound (i.e., the mandrelor glans) per the following equation:

$\begin{matrix}{\underset{\underset{In}{Conduction}}{{\mathrm{\Upsilon}\frac{^{2}{T_{1}(x)}}{x^{2}}} -}\pi \; h_{e}D\underset{\underset{Out}{Convection}}{\left\lbrack {{T_{1}(x)} - T} \right\rbrack +}\left( \frac{I_{1}^{2}R_{1,0}}{L_{1}} \right)\underset{\underset{In}{{Electrical}\mspace{14mu} {Power}}}{\left\lbrack {1 + {\alpha \left( {{T_{1}(x)} - T_{0}} \right)}} \right\rbrack = 0}} & (1)\end{matrix}$

where x is the axial dimension of the heated length; γ is the overallaxial conductance (kA); D is the outside diameter of the velocity probe;T is the gas temperature; I₁ is the measured electrical current suppliedto the winding; R_(1, 0) is the electrical resistance of the winding atreference temperature T₀; L₁ is the length of the winding (and heatedlength); and α is the temperature coefficient of resistance of thewinding.

Notably, Equation (1) is related to the differential equation derived byBruun (Bruun, H. H. 1995. Hot-Wire Anemometry: Principles and SignalAnalysis. Oxford: Oxford Univ. Press.) for a hot-wire anemometer.However, in relation to the Bruun equation, Equation (1) substitutes an“effective” film coefficient h_(e) for the classical film coefficient hin the original, expressed as:

$\begin{matrix}{h_{e} = \frac{h}{1 + {h\; \pi \; {DL}_{1}R_{skin}}}} & (2)\end{matrix}$

where R_(skin) is the electrical analog of thermal resistance forvarious layers of “insulation” over the heated length. Further, h_(e) isderived from the convective heat transfer rate Q₁ from the controlvolume, or heated length, as shown in the following equation:

Q ₁ ×h(πDL ₁)(T _(e−) T)=h _(e)(πDL ₁)(T ₁ —T)   (3)

where T_(e) is the average temperature of the external surface of thevelocity sensor probe, and T₁ is the average temperature of T₁(x) overlength L₁. Skin resistance R_(skin) lowers external temperature T_(e) ofthe control volume according to the equation:

T _(e) ×T ₁ −Q ₁ R _(skin)   (4)

Together, these equations are used to solve for mass flow rate asfurther described.

In order to run the equation set as part of an effective computationmodel, the subject hardware must conform to the following assumptions:(a) that the temperature distribution is relatively one-dimensional inthe independent variable x; (b) that a second differential equation forthe temperature distribution of the housing shell is not required; and(c) that γ, h_(e), and R_(skin) are constant at their average valuesover length L₁. As for assumption (a), this may hold true with hardwarewhere L₁/D is sufficiently large (e.g., at least about 3:1 and morepreferably about 4:1 or more). However, the ratio can be less than thatnormally required for the one-dimensionality assumption to apply.Namely, in the constant temperature mode of operation, the entireoutside surface (e.g., a cylindrical surface) of the control volume ismaintained at constant average temperature T₁ and the RTD windingmaintains the entire circumferential surface of each differential slice(at a given axial location x) of the control volume at essentially thesame temperature (i.e., T₁(x)). As such, the entire slice does not vary(or only negligibly so) with the radial or azimuthal dimensions (incylindrical coordinates), varies only with the axial dimension x, andhas a temperature T₁(x) throughout.

With these assumptions in mind, then the following exponential solutionfor Equation (1) can be applied:

$\begin{matrix}{{{{T_{1}(x)} - T} = {{B_{1}^{\beta \; x}} + {B_{2}^{{- \beta}\; x}} + \frac{g}{\beta^{3}}}}{{where}\text{:}}{\beta = {\left\lbrack {\frac{\pi \; h_{e}D}{\mathrm{\Upsilon}} - \frac{\alpha \; I_{1}^{2}R_{1,0}}{\mathrm{\Upsilon}\; L_{1}}} \right\rbrack^{\frac{1}{2}}\left( m^{- 1} \right)}}{S = {{\left( \frac{I_{1}^{2}R_{1,0}}{\mathrm{\Upsilon}\; L_{1}} \right)\left\lbrack {1 + {\alpha \left( {T - T_{0}} \right)}} \right\rbrack}\mspace{14mu} \left( {K\text{/}m^{2}} \right)}}{B_{1},{B_{2} = {{Constants}\mspace{14mu} {(K).}}}}} & (5)\end{matrix}$

Associated with an analysis employing this solution, in cases where aseparate temperature sensor probe is included in the system, thedifferential equation used in the analysis for the classical case ofheat transfer from fins may be employed. As such, the performance of thetemperature sensor probe may be characterized according to:

$\begin{matrix}{{\frac{^{2}{T_{temp}(x)}}{x^{2}} - {\beta_{temp}^{2}\left\lbrack {{T_{temp}(x)} - T} \right\rbrack}} = 0} & (6)\end{matrix}$

in which Equation (6) has a well known exponential solution per:

$\begin{matrix}{{{{{T_{temp}(x)} - T} = {{C_{1}^{\beta_{temp}x}} + {C_{2}^{{- \beta_{temp}}x}}}}{{where}\text{:}}\beta_{temp} = {\left\lbrack \frac{\pi \; h_{temp}D_{temp}}{\mathrm{\Upsilon}_{temp}} \right\rbrack^{\frac{1}{2}}\left( m^{- 1} \right)}}{C_{1},{C_{2} = {{Constants}\mspace{14mu} (K)}}}} & (7)\end{matrix}$

and the constant coefficients are determined by boundary conditionsprovided by temperature data where two temperature sensors are includedin the probe. As such, temperature sensor spacing is advantageouslymaximized to offer greater temperature spread, and thus, resolution incomputed output.

So-optimized, one invention embodiment concerns a system that isconfigured to run the equations and output any of gas temperature andmass flow rate in response to sensor measurements and/or input pressurefor a given gas (after calibration with a surrogate gas such as air) byreference to a library of properties for others.

Typically, the equations are solved in an iterative, converging methodtaking the closest approximation of gas temperature (e.g., from thedistal sensor in a temperature sensor probe, or—if not available—from orrelated to the distal temperature sensor measurement in a 3-sensorvelocity sensor probe) as the “seed” value in connection with othercommonly-used formulae describing Reynolds, Nusselt and Prandtl numbers.So that the calculated solution offers sufficient accuracy, the hardwareis configured to conform to the assumptions required above and is alsopreferably implemented in connection with “dry” sensor technology asnoted above. Accordingly, inventive aspects cover the requisitehardware.

According to other embodiments hereof, computer readable media withinstructions stored thereon implementing the solution method describedherein may be provided. Such computer readable media may be implementedon a general purpose computer (e.g. as software or executableinstructions stored on a recordable type media such as a hard diskdrive, digital tape, compact disc or the like), or viaApplication-Specific Integrated Circuit (ASIC) or other hardware means.Furthermore, the computer readable media embodying aspects of theinvention may advantageously be used in conjunction with the sensorconfigurations in any suitable combination and may be used to obtainflow measurements in real time. By “real time”, it is generally meant inthe context of this disclosure, that calculations performed by a chipsetexecuting instruction according to the present solution may involveoutputting and/or updating a result about every second, or in someexamples up to about 5 seconds. Moreover, to be of use in monitoring anindustrial process, the real time output should be continuous (i.e.,delivered over a duration without interruption—on the order of hours andeven days or more).

In a four-temperature sensor configuration as illustrated in thedrawings with spacing between the secondary temperature sensor and theheated length in the velocity sensor probe, the mathematics employed maysolve the differential equation—Equation (1)—in connection with one ormore intermediate nodes. With another configuration in which a secondarysensor is immediately adjacent (i.e., touching) the proximal end of theheated length, intermediate node(s) is/are eliminated and the secondarytemperature sensor may directly provide a proximal boundary conditionfor the differential equation solution. With another secondary sensorimmediately adjacent the distal end of the heated length, the otherboundary condition may be directly provided for that end of the heatedlength. Alternatively, a distal secondary sensor may be provided in thevelocity sensor probe and calculations may optionally be made using oneor more intermediate nodes.

Interestingly (whether employed at some distance in conjunction withnodal analysis modeling or located immediately adjacent), use of thethird temperature sensor (i.e., a second non-heated sensor) in thevelocity sensor probe permits the elimination of the temperature sensorprobe altogether. As alluded to above regarding the discussion of gastemperature seed value(s), given knowledge of the average temperature ofthe heated sensor and its end/boundary conditions (via the secondarysensor(s)), the seed values for the temperature of flowing gas can beinferred.

Notably, for such purposes, it may actually be preferred to separate thedistal secondary sensors some distance from the heated sensor. By doingso, a greater temperature difference can be measured, thereby improvingthe accuracy of the derived gas temperature value. A distance betweenabout two-to-three times the diameter of the velocity sensor probe (orthat of the heated sensor windings) between the heated sensor and thedistal secondary sensor may be used for such purposes. Other distancesexceeding two or three times the diameter of the velocity sensor probemay be used if desired, and the length of the velocity sensor probe maybe so configured as to accommodate such distances.

Any and all of these hardware configurations are intended as embodimentsof the present invention as well as the software methodology associatedwith their use. Moreover, it is to be appreciated that not allvariations of the invention are practiced with an outer housing shell.

Still further, the assemblies described above may be configured inconnection with relevant hardware for use as an insertion or as anin-line type flow meters. In some embodiments, complete mass flow metersinclude separate fluid temperature and velocity sensor probe elements.In the three-sensor velocity probe sensor variation, a complete massflow meter assembly may utilize only one probe (i.e., the velocitysensor probe).

BRIEF DESCRIPTION OF THE DRAWINGS

The figures diagrammatically illustrate aspects of various embodimentsof different inventive variations.

FIGS. lA and 1B are in-line and insertion type configurations,respectively, with installed sensors as may be employed in embodimentsof the present invention.

FIGS. 2A and 2B are flow-axis/direction and end-view details,respectively, of the same sensor hardware.

FIG. 3 is a partial section view of a known sensor configuration.

FIG. 4A is a partial section view of a sensor configuration according toone example of the present invention.

FIG. 4B is partial section view of sensor configurations according toanother example of the present invention.

FIG. 5 is an oblique construction view of a velocity sensor probecorresponding to the embodiment shown in FIG. 4A.

FIGS. 6A-6C are side, distal and proximal views, respectively, of avelocity sensor probe construction corresponding to the embodiment inFIG. 4B.

FIGS. 7A-7C are partial side section, distal and proximal views,respectively, of a velocity sensor probe construction corresponding tothe embodiment in FIG. 4B.

FIG. 8 is a block diagram of suitable electronics hardware for carryingout software operations as variously described.

FIGS. 9A-9C are charts illustrating operation according to embodimentsof the invention.

FIGS. 10A-10C are charts illustrating measured vs. actual mass velocityfor air and methane, respectively, accomplished with an embodiment ofthe present invention.

Variations of the embodiments shown in the figures are contemplated, andshall be considered within the scope of the claimed invention(s)explicitly, or under the Doctrine of Equivalents.

DETAILED DESCRIPTION

Thermal dispersion mass flow meters may generally be implemented in twoprimary configurations: in-line and insertion. FIGS. 1A and 1Brespectively, show examples of these two configurations and their majorcomponents. In FIG. 1A, the mass flow meter assembly 100 is shownconnected with an adapter 10 extending from pipe 12. Because thevelocity sensor probe element 20 and the temperature sensor probeelement 30 are intended to be enclosed within the pipe 12 as a deliveredunit for in-line placement within a system, the sensor probe elements donot require a protective shield. The In-line mass flow meter assembly100 is typically attached to the process piping 14 by means of flanges16, 16′. The mass flow meter assembly 100 may also include one or moretwo perforated flow plates 18 provided in series and upstream of thevelocity and temperatures sensor probe elements to smooth outdisturbances and/or turbulence in the flow reaching the sensor probeelements.

An insertion type mass flow meter assembly 100′ may include some of thesame components as the mass flow meter assembly 100, and in addition mayinclude a shield element 40 as the meter 100′ is not delivered enclosedin a pipe but configured to be inserted into the process pipe 14. Boththe in-line and insertion mass flow meters (e.g., 100 and 100′respectively) may also include electronics enclosed in electronicshousing 110, which may include a digital readout display 112. Thedisplay 112 may be coupled to one or more processor and/or otherelectronics and configured to display signals corresponding tomeasurements and/or mass flow rate results calculated by the one or moreprocessors. The electronics housing 110 may fully enclose theelectronics necessary for performing computations, as will be described,and may include a variety of suitable electronic components, includingbut not limited to processors, storage, communication devices,input/output devices, and the like.

In many insertion-type and in-line configurations of mass flow meters,the velocity sensor and temperature sensor probe elements are alignedsubstantially perpendicular to the main fluid flow stream (F) as shownFIGS. lA and 1B. However, in-line mass flow meter arrangements, as maybe employed in connection with the teaching herein, may alternativelyhave their sensor probe elements arranged axially to the flow (e.g.,with a longitudinal direction of the sensor probe elements disposedsubstantially along the direction of the flow).

In-line flow meters are typically applied to pipes and ducts withdiameters typically ranging from about 10 to 100 mm (0.25 to 4.0 inchpipe sizes), but some manufacturers offer sizes up to 300 mm (12.0 inchpipe size). Process connections include flanges, pipe threads, andcompression fittings. Insertion flow meters usually are applied tolarger pipes, ducts, and other flow conduits having equivalent diameterstypically ranging from approximately 75 mm to 5 m.

Because insertion meters are more economical than in-line meters, theyalso have found wide use as flow switches. Compression fittings andflanges are commonly used process connections. Insertion meters measurethe mass velocity at a point in the conduit's cross-sectional area. Forapplications with smaller conduits, they may be flow calibrated tomeasure the total mass flow rate through the conduit.

Multipoint insertion meters measure the mass velocities at the centroidsof equal areas in the cross section of large pipes, ducts, and stacks.The total mass flow rate through the entire conduit is the average massvelocity of the several points multiplied by the total cross-sectionalarea and the standard mass density of the gas.

Any of such technologies/approaches may be employed in connection withthe flow meters described herein. More specifically, FIGS. 2A and 2Bshow a flow sensor that is common to both in-line and insertionconfigurations comprising a housing 50 with an extension region 52 fromwhich velocity sensor and temperature sensor probes 20, 30 extend alongwith shield “legs” 42—although in smaller in-line meters the flow sensormay not have a shield. Notably, housing 50 incorporates an open-endedshield 40 design and a shoulder 54.

Traditional insertion meters have a shield with a closed end that cancause the flow over the velocity sensor probe to be non-uniform andturbulent. The open-ended shield shown still protects the sensors butdoes not have this problem. In addition, the length of reduced diameterof extension 52 and shoulder 54 just above the flow sensor redirects andturns axial flow downwash so it flows circumferentially around the probebefore it can pass over the velocity sensor probe, thereby minimizinganother source of inaccuracy.

The purpose of thermal dispersion mass flow meters is to make anundistorted measurement of the free-stream velocity of a fluid justupstream of its position. Thus, the flow sensor components should notthemselves create problematic flow disturbances or turbulence in thevelocity field before it has passed over the velocity probe and issensed. Features of the design of the flow sensor in FIGS. 2A and 2Baccomplish this purpose. The aforementioned shoulder 54, open-endedshield 40 and shielded legs 42 (with their aerodynamic cross section)mitigate deleterious flow disturbances from the housing and shield. Inaddition, the location of the velocity sensor probe 20 set forward(relative to the flow) and relatively more centered within the shieldavoids flow interaction with the shield and/or temperature sensor probe30 (which is itself located downstream of, and offset from, the velocitysensor probe).

Referring now to FIG. 3, a conventional thermal dispersion flow sensor200 is shown, which may be used in an in-line and/or an insertion typemass flow meters intended for industrial-grade applications. The flowsensor 200 includes a velocity sensor probe 220 and a temperature sensorprobe 230. The velocity sensor probe 220 has an electrically self-heated(or heatable) temperature sensor element 222 located in its tip thatboth heats the velocity sensor probe and measures its own averagetemperature. The temperature sensor probe 230 has a single non-heatedtemperature sensor element 232 located in its tip that measures thetemperature of the gas in which the flow sensor 200 is immersed. Becauseflow sensor 200 has a total of two temperature sensing elements (one ineach probe element 220,230), it is often called a “two-temperature” flowsensor. However, as previously described and as will be furtherdiscussed, the flow sensor 200 may suffer numerous shortcomings,including problems associated with skin resistance and stem conduction.

The velocity sensor and the temperature sensor probes are mountedside-by-side in a sensor housing assembly 210. Each sensor is enclosedin a rugged, sealed, single-ended, corrosion-resistant metallic tube212. In traditional velocity sensors of the kind shown in FIG. 3, thetemperature sensor element 222 is potted into the tip of the tubularsheath. Typically, the potting, or filler material (not shown) isceramic cement or epoxy. Heat sink grease also has been used for thispurpose.

In use, the velocity sensor probe and the separate temperature sensorprobe of the flow sensors illustrated are inserted or immersed in theflow stream. For that reason, thermal dispersion mass flow meters arealso often called “immersible” thermal mass flow meters. Notably, theoutside temperature external to the flow sensor may be different thanthe gas temperature in the flow conduit. For that reason, heat can beconducted in or out of the stems of the velocity sensor and thetemperature sensor probes. In the field, the heat so-conducted througheach stem may be different from its value at the time of flowcalibration, for example if the outside temperatures or other parametersor conditions in the field are different than during calibration.Additionally, heat can be conducted from the hot velocity sensor probe220 to the cooler temperature sensor probe 230 via their stems (i.e.,the tubes 212 together with any internal components therein includingthe housings, wires, ferrules/spacers, etc.). These effects are furthercomplicated because they depend on the mass flow rate. Left uncorrected,the associated stem conduction constitutes a major source of error inmeasuring mass flow rate.

Mass flow meters according to embodiments of the present invention mayaddress some or all of these problems, for example the problem of stemconduction. In this regard, mass flow meters according to the presentdisclosure may include three or more temperature sensing elements forimproved accuracy and ease of use, as will be further described. In someconfigurations a total of four temperature sensing elements may beincluded, with two elements positioned in the velocity probe and twoelements in the temperature probe. In other examples, the flow sensormay not include a temperature probe, and instead three temperaturesensing elements may be arranged, as will be described, in a singleprobe which are configured to perform all of the functionality of theflow sensor. Specific relative arrangement of the temperature sensingelements within each probe may be used so as to facilitate the use ofimproved algorithms for calculating certain measurements, also describedherein.

An example of a thermal dispersion flow sensor 200′ is shown in FIG. 4A.The thermal dispersion flow sensor 200′ includes a velocity sensor probe20 and a temperature probe 30 according to examples of the presentdisclosure. The velocity sensor probe 20 may have a diameter D and thetemperature sensor probe 30 may have a diameter D_(T), which diametersmay or may not be equal. Also, each of the probes 20 and 30 may includea plurality of temperature sensing elements (e.g. elements 22, 24 and32, 34) therein. The velocity probe 20 in the example in FIG. 4Aincludes two temperature sensing elements 22, 24, and the temperatureprobe 30 includes two temperature sensing elements 32, 34. For higheraccuracy and higher stability, each of the temperature sensing elements,22, 24 and 32, 34 may be resistance temperature detectors (e.g.,platinum RTD sensors) that may be provided as thin film or wire-woundRTDs and which may be protected by a thin insulation layer of glass orceramic. As may be known, the electrical resistance of RTDs increases astemperature increases thus providing a means for transducing/translatingtheir electrical output into temperature. Other types of temperaturesensing elements, such as thermistors, thermopiles, thermocouples, andmicro-electronic machined devices, may also be used in place of or incombination with RTDs, for example for applications with lower accuracyrequirements.

Another example of a thermal dispersion flow sensor 200″ is shown inFIG. 4B, which may offer additional functionality, as well as certainadditional flexibility in deployment configuration. The flow sensor 200″may include a velocity sensor probe 20′ with a diameter D and atemperature probe 30. The velocity sensor probe 20′ in this example mayinclude three separate temperature sensing elements 22, 24, and 26synergistically arranged as will be described below.

In each of the examples depicted in FIGS. 4A and 4B, the velocity sensorprobes 20, 20′ include a heated sensor 22 which is preferablyimplemented using a platinum wire-wound RTD construction. Used in eithera constant current or constant temperature mode, the temperature sensor22 may be referred to as a “heated” sensor to designate its physical andassociated electrical character, whether or not in use. That is, theterm “heated” does not imply that the sensor 22 is heated at all times,particularly when not in use. The velocity sensor probes 20, 22′ mayalso include a second (or secondary) temperature sensor 24. Thesecondary sensor 24 may be implemented using a thin-film platinum RTD(TF RTD). Other ones of the temperatures sensing elements (e.g. sensors34, 32, and 26) may also be implemented using TFRTD sensors. Suchsensors are not only compact for deployment; they may also be relativelyinexpensive, while capable of holding excellent tolerances. Secondarysensor 24 is typically not self-heated, but may instead be used tomeasure temperature.

In the example in FIG. 4A, the heated sensor 22 (e.g., wire-wound RTD)may have a proximal end and a distal end, which define a heated length(L₁ as in FIG. 5) of the sensor 22. The secondary sensor 24, which maybe used for the purpose of compensating for stem conduction, may belocated near the proximal end of the heated length in order to provideaccurate boundary condition for the algorithm described below.Specifically, the distance x_(22,24) separating the active region ofsensor 24 and the proximal end should be less than or equal to aboutthree times the diameter of the velocity sensor probe D. In someexamples, and as shown in FIG. 4A, sensors 22, 24 may be set moreclosely than three times the diameter D, including being directlyadjacent (i.e., touching, nearly touching or even overlapping) oneanother. That is, in some examples, it may be advantageous to minimizethe distance x_(22,24) between heated sensor 22 and senor 24. Incontrast, with respect to arrangement of sensors 32 and 34 in thetemperature probe 30, it may be advantageous, for example for purpose ofmeasured temperature range separation, to instead maximize the distancebetween said probes x_(32,34.) For example, an active region of thedistal sensor 32 and an active region of the proximal sensor 34 may beseparated by a distance (e.g. distance x_(32,34)) of at least 2 timesthe diameter of the temperature probe D_(T). In other examples, agreater than two times D_(T) may be used, for example a distance ofthree times or four times the diameter. Virtually any distance dependingon the length of the probe 30 may be used, provided the proximal sensor34 is located within the flow within which the probe 30 is immersed.

As shown in FIG. 4B, another temperature sensor 26, which may be athin-film RTD, may be included in the velocity sensor probe 20′. Theactive region of sensor 26 may be separated from a distal end of thewire-wound sensor 22 (e.g. heated sensor 22) by a distance x_(22,26).The amount of separation of these sensors (e.g. the distance x_(22,26)between heated sensor 22 and distal sensor 26) may be selected to bestserve the purposes described herein. An example of one option isindicated by the dashed-line extension of housing shell 28 and moredistal location of sensor 26′.

As for the different purposes: when set directly adjacent to one another(i.e., touching/overlapping) intermediate nodes (used in conjunctionwith the aforementioned “nodal analysis”) can be eliminated in systemanalysis as described above; with marginally more separation (e.g.,about one diameter) such analysis may utilize several nodes; set furtherapart (e.g., separated by about 2 to 3 diameters) additionalcomputational intensity may be required, but the “velocity” sensor probe20′ will be better able to determine gas temperature with sensor26—enabling the elimination of a separate “temperature” sensor probe. Inthe last instance, probe 20′ might alternatively be referred to as a“universal” or “independent” mass flow sensor probe.

However configured, in the examples shown in FIGS. 4A and 4B, thewire-wound platinum RTD sensors 22 according to the present examplestypically have a resistance ranging from about 10 to about 30 Ohms Thetemperature sensing elements implemented as thin-film RTDs (e.g. sensors24, 26, 32, 34) may have resistances ranging from about 500 to about1000 Ohms.

FIG. 5 illustrates an example of a construction of velocity probe 20.Here, housing shell 28 encases/encloses sensors 22 and 24 as illustratedbefore. Visible in more detail, is the manner in which platinum RTD wire60 is formed in multiple turns around a mandrel (e.g., alumina) 62. Thecoiled length L₁ of the heated winding 60 is the length of theaforementioned “control volume” or “heated length” discussed inconnection with Equation (1) above. The mandrel (alternatively referredto as a “glans” by those with skill in the art) includes horizontalslots 64 (obscured slot not shown) for Pt wire access into the center ofthe body. Electrical leads A” connected to the heated sensor wire 60 arealso shown (whereas those to/from “secondary” sensor 24 are not).

This configuration of the glans is well known. New, however is theplacement of a secondary temperature sensor 24 adjacent to mandrel/glans62. As shown, it is touching the body over which the wire is wound. Assuch, a very predictable estimate to establish a gap G between theposition of a proximal end of heated sensor 22 and the activemeasurement area/point of sensor 24 can be established using FEAanalysis.

Still, the proximal extent of the mandrel separates the active region ofsensor 24 from the proximal end of windings 60 and—thus—the proximal endof the heated sensor 22. The configuration in FIGS. 6A-6C and 7A-7C areadapted to enable closer placement of the included sensors 22, 24 (in atwo-sensor variation, not shown) or 22, 24 and 26 (in the three-sensorvariations, shown) to enable elimination of FEA node(s) in systemanalysis by directly using the temperature sensor measurement(s) asboundary condition(s) in the aforementioned mathematical modeling.

More specifically, the FIG. 6A-6C glans element 62′ incorporates one ormore slots 70 cutting across the body. The slot(s) can be machined orotherwise formed (e.g., water jet cutting). In any case, it/theyprovide(s) clearance for the secondary temperature sensor(s) 24, 26.

As an alternative, the approach in FIGS. 7A-7C includes counter-bores orrecesses 72 serving the same purpose. In such systems, the closestsurface of the temperature sensor body/bodies may be touching orinserted within a region of the mandrel over which the wire-wound heatedsensor is formed.

Naturally, in three dimensions, there is radial separation of thecomponents. But with conformance to the hardware configurationassumptions above, the one-dimensionality of the mathematical analysiscan be maintained. Thus, in relevant part (i.e., x dimension along thelength), the spacing is preferably from about 0.05 inches to about zerox (projected) distance from the last turn of the winding (with a smallinset/overlap tolerance of about 0.02 inches), or, even more moderately,about 0.10 inches to about zero distance (with an inset/overlap of about0.03 inches).

If using a distal secondary temperature sensor in the velocity sensor(as shown in each of FIGS. 6A-6C and 7A-7C), glans 62′/62″ willtypically include four lumen, providing insulated clearance holes forthe Pt winding (i.e., heated sensor 22) leads “A” and distal TFRTD(i.e., secondary sensor 26) lead wires “B.” Electrical leads “C” for theproximal TFRTD (i.e., secondary sensor 24) extend proximally withoutneed for clearance holes through the mandrel/glans.

An overview of optional system-level constructional details arepresented in FIG. 8. The block diagram illustrates amicroprocessor-based thermal dispersion mass flow meter with thefour-temperature flow sensor configuration shown in FIG. 4A. Adaptationto other sensor configurations as discussed herein should be within theability of those with ordinary skill in the art.

In any case, the components shown may be set within housing 110, alongwith provision for wired connection or wireless data transmission (suchas though Bluetooth, WiFi, etc.). The voltage sensing wires “D” make themeasurement of the RTD resistances independent of the length of the flowsensor cable, facilitating remote location of the transmitter.

With the velocity sensor operated in the constant temperaturedifferential mode, heating current I₁ depends on the electricalresistance R₁ of the velocity sensor 22 and the electrical power input Wrequired to maintain ΔT constant (i.e. the difference in temperaturebetween the gas and average temperature of the heated sensor). Wtypically ranges from about 0.1 to 5 watts depending on the “overheat”ΔT, the mass flow rate, and the size of the velocity sensor. Thetemperature sensing current I₂ is held constant and is less than 1 mA toavoid self-heating sensor 24. The “Analog layer” shown includesprecision resistors for measuring the currents I₁ and I₂ but has nobridge circuit. Analog-to-Digital conversion is provided with the “A/D”converter between the Analog layer and a Microprocessor (optionally withon-board ROM, EPROM or other computer readable medium storinginstructions). The system (i.e., by calculations preformed by theMicroprocessor in accordance with instructions) digitally linearizes themass flow rate q_(m) output and (optionally) T and P outputs, providinganalog outputs for these variables.

The system may further include algorithms based on the above principleof operation that manage changes in gas selection, gas temperature, andgas pressure in connection with measurements received from theassociated hardware. The system may also provides a selection of digitalcommunication protocols, including Hart, Foundation Fieldbus, andProfibus (all trademarked). Likewise, the systems described herein mayenable a number of traditional digital functions, such as: amulti-variable digital readout and user interface; digital RS 232 and RS485 communications; flow meter diagnostics, validation, calibrationadjustment, and reconfiguration; flow totalization; and alarms.

Certainly, the thermal anemometers according to examples of theinvention retain advantageous performance if operated with eitherdigital or analog sensor-drive electronics, or with a combination ofboth, in either the constant-temperature or constant-current modes ofoperation. Digital electronics may be preferred for reason of simplifiedcomputations based on heat-transfer correlations and correctivealgorithms, that compensate for any changes (e.g., as referenced to flowcalibration conditions) in the fluid itself, fluid temperature, fluidpressure, ambient temperature and other variables and influenceparameters, thereby yielding higher system accuracy. Said heat-transfercorrelations and corrective algorithms may be based on known empiricalheat transfer correlations, specific experimental data for the thermalanemometer of the present invention, a gas property library inelectronic memory, physics-based heat transfer theory, and othersources.

With a system as described in connection with the above, examples of howa four-temperature microprocessor-based system manages changes in gasselection, gas temperature and gas pressure for air, methane, and argonare provided in FIGS. 9A-9C. These figures are plotted in theconventional manner with the mass velocity V_(s) shown as theindependent variable and the electrical power W shown as the dependentvariable, whereas in the system they have reversed roles. The threefigures reflect the strong direct dependence the electrical power W hason the thermal conductivity of the gases. Thus, FIG. 9A results from thefact that k_(methane)>k_(air)>k_(argon) and FIGS. 9B and 9C result fromthe fact that thermal conductivity increases as gas temperature andpressure increase, respectively. The fact that thermal conductivity, andtherefore W, increases with gas pressure as shown in FIG. 9C is aphenomenon that has heretofore been ignored, but for higher accuracyapplications should be included.

FIGS. 9A-9C also reveal the non-linear, logarithmic nature of theoutput. A log vs. log plot of these figures (not shown) demonstrates anearly straight line over approximately 1 to 150 standard m/s. Thislogarithmic property is responsible for the exceptional rangeability andlow-velocity sensitivity of thermal dispersion mass flow meters. Arangeability as high as 100:1 is common. Even higher rangeabilities areachieved with multi-range flow calibration. Detectable minimum pointmass velocities as low as approximately 0.1 standard m/s (approximately20 standard ft/min) are reported by some manufacturers.

FIGS. 10A-10C show further results of the four-temperaturemicroprocessor based system. FIG. 10C reveals how the temperaturedistribution T₁(x) of the heated velocity sensor 22 (as shown in FIG. 5)undergoes major changes as the mass velocity V_(s) increases from 0 to100 standard m/s. FIGS. 10A and 10B show, for air and methane, thesuperb comparison between results calculated using the four-temperaturemicroprocessor-based system and actual flow calibration data.Comparisons for other gases are likewise excellent.

Variations

Exemplary aspects of the invention, together with details regardingmaterial selection and manufacture have been set forth above. As forother details of the present invention, these may be appreciated inconnection with the above-referenced patents and publications as well asis generally known or appreciated by those with skill in the art. Thesame may hold true with respect to method-based aspects of the inventionin terms of additional acts as commonly or logically employed. Regardingsuch methods, including methods of manufacture and use, these may becarried out in any order of the events which is logically possible, aswell as any recited order of events. Furthermore, where a range ofvalues is provided, it is understood that every intervening value,between the upper and lower limit of that range and any other stated orintervening value in the stated range is encompassed within theinvention. Also, it is contemplated that any optional feature of theinventive variations described may be set forth and claimedindependently, or in combination with any one or more of the featuresdescribed herein.

Though the invention has been described in reference to severalexamples, optionally incorporating various features, the invention isnot to be limited to that which is described or indicated ascontemplated with respect to each variation of the invention. Variouschanges may be made to the invention described and equivalents (whetherrecited herein or not included for the sake of some brevity) may besubstituted without departing from the true spirit and scope of theinvention.

Reference to a singular item includes the possibility that there are aplurality of the same items present. More specifically, as used hereinand in the appended claims, the singular forms “a,” “an,” “said,” and“the” include plural referents unless specifically stated otherwise. Inother words, use of the articles allow for “at least one” of the subjectitem in the description above as well as the claims below. It is furthernoted that the claims may be drafted to exclude any optional element. Assuch, this statement is intended to serve as antecedent basis for use ofsuch exclusive terminology as “solely,” “only” and the like inconnection with the recitation of claim elements, or use of a “negative”limitation.

Without the use of such exclusive terminology, the term “comprising” inthe claims shall allow for the inclusion of any additionalelement—irrespective of whether a given number of elements areenumerated in the claim, or the addition of a feature could be regardedas transforming the nature of an element set forth in the claims. Exceptas specifically defined herein, all technical and scientific terms usedherein are to be given as broad a commonly understood meaning aspossible while maintaining claim validity. The breadth of the inventivevariations is not to be limited to the examples provided and/or thesubject specification, but rather only by the scope of the claimlanguage.

What is claimed is:
 1. An apparatus for use in a mass flow meter forimmersion in fluid flow comprising: a Resistance Temperature Detector(RTD) comprising a wire winding set upon a non-conductive spacer anddefining having a heated length; and a proximal temperature sensorreceived within the spacer.
 2. The apparatus of claim 1, furthercomprising an elongate temperature sensor.
 3. The apparatus of claim 1,further comprising a housing shell for the RTD and proximal temperaturesensor.
 4. The apparatus of claim 1, further comprising a distaltemperature sensor received within the spacer.
 5. The apparatus of claim1, wherein a surface of at least one of the proximal and distaltemperature sensors is located within about 0.1 inches of the heatedlength of the RTD.
 6. The apparatus of claim 5, wherein the surface ofat least one of the proximal and distal sensors and the heated length ofthe RTD overlap.
 7. The apparatus of claim 1, further comprising acomputer processor for outputting mass velocity measurements in realtime.
 8. The apparatus of claim 7, further comprising a transmitter forwireless communication of the measurements.
 9. The apparatus of claim 1,wherein the RTD comprises a platinum wire winding.
 10. The apparatus ofclaim 1, wherein the spacer is slotted or recessed at a proximal end toreceive the proximal temperature sensor therein.
 11. The apparatus ofclaim 10, wherein the proximal temperature sensor comprises a thin-filmRTD.
 12. The apparatus of claim 4, wherein the spacer is slotted orrecessed at a distal end to receive the distal temperature sensortherein.
 13. The apparatus of claim 12, wherein the distal temperaturesensor comprises a thin-film RTD.
 14. The apparatus of claim 4, whereinthe spacer includes four lumen with two lumen receiving electrical leadsfor the wire-wound RTD and two lumen receiving electrical leads for thedistal temperature sensor.